The Fabrication And Investigations On Stripe-Loaded Waveguides And Novel Nano-Materials

The integrated photonics, which refers to the fabrication and integration of several photonic components on a common planar substrate, is a swiftly developing and synthesized technical domain ever since the end of the twentieth century. Optical waveguide is the basis of the devices in integrated optics field, as signal propagation channels and equipment connected some devices with others. In geometry, waveguides could be divided into two types:one-dimensional waveguides (planar waveguides) and two-dimensional waveguides (channel or ridge waveguides). The flexibility in geometry configuration and high integration level, and make optical waveguides indispensable components in integrated optical systems. In addition, stripe-loaded waveguides, as a newly structured waveguides, has attracted great research interest in the field of integrated. A stripe-loaded waveguides includes two parts:the planar waveguides with the higher index material and a stripe of slightly lower index material as the loading. Since, within the region of the strip, most of the energy is confined in the film, requirements on the edge roughness of the strip are no longer as severe as in rectangular film waveguides and therefore strip-loaded structures seem easier to fabricate. In addition, using the acousto-optic, electro-optic, and nonlinear properties, functional photonics devices can be achieved based on waveguide structures. The quality of optical device is determined directly by the optical waveguides; the fabrication of low loss waveguides and the investigation of optical properties of waveguides are a continuous interesting topic in integrated optics.Lithium niobate (LN) has been one of the most attractive materials due to its outstanding electro-optic, acousto-optic, and nonlinear properties, etc. In recent years, single-crystal LN thin film on insulator (LNOI), produced by bonding techniques, offers an ideal platform in modern integrated optics. Due to its outstanding physical properties and high-refractive-index contrast between LN and SiO2, good confinement of light, the integration density of the integrated photonic circuits and the performance of the photonic devices based on LNOI have been significantly improved. At present, various ultra-compact photonic integrated devices such as electro-optical modulators, micro-disk resonators, and photonic crystals resonators have been reported. LNOI is developing rapidly, as one of the most valuable new materials in integrated optics.Up to now, several manufacturing methods have been employed to fabricate waveguide structures, including energetic ion beam implantation, ion exchange, thin film deposition, metal ion thermal diffusion and pulsed laser inscription, etc. Metal thermal oxidation technique is an effective and simple method of fabricating waveguide structures, substrate loaded with metal film is oxidized in a furnace with flowing oxygen at high temperature condition, thus the waveguide structure is formed with metal oxide thin films and substrate. This method is very simple, metal oxide thin film fabricated by Metal thermal oxidation method is denser and high content of oxygen. Combined with lithography, Metal thermal oxidation method can fabricate various structure waveguide. In this dissertation, we have been fabricated the stripe-loaded waveguides in lithium niobate single-crystal thin film by Metal thermal oxidation method.Mode Solution is a combination eigenvalue mode solver and wave propagator that accurately simulate structures that support guided modes, and an ideal tool with design, analysis and optimization of the waveguide structure. For arbitrarily shaped waveguide, physical properties of guided modes can be analyzed by eigenvalue mode solver and wave propagator. The 2.5D propagator accurately describes the propagation of light in planar integrated optical systems, from ridge waveguide-based systems to more complex geometries such as photonic crystals and ring resonators etc. The Eigen mode expansion propagator allows the propagation of the long optical devices, which is useful for waveguide couplers, long tapers and other devices. In this dissertation, we have been simulated the propagation properties of the waveguides.Porous nano-cages with a controllable interior space and a distinctive porous shell have received increasing attention in recent years. The unique object with low adsorptive capacity in UV-Vis-IR region has the significant application in novel optical waveguide device. On the other hand, porous nano-cages are characterized as fine nano-particles with lower densities, larger specific surface area, which have received increasing attention as super-active catalysts, faster responsive sensors, light-emitting devices, and biomedical sensors, etc. Since the properties of the nano-cages are strongly dependent on their porous surfaces and hollow interiors, a simple, versatile and rapid route to controllably synthesize nano-cages with designed shape and geometry is great significance for the specific applications. Laser ablation of bulk target in activated liquid as an attractive green technique for the fabrication of novel nano-structures because of the highly non-equilibrium processing (unique hot plasma-104℃, high pressure～10 Ga surrounding under the condition of the liquid confinement, the higher ionization process) and without complicated purification procedures. In this dissertation, we have been fabricated the novel nano-materials by laser ablation in solution.In this dissertation, we report fabrication, characterization and application of waveguide. The metal thermal oxidation method was used to fabricate planar and strip-loaded waveguide in lithium niobate and LNOI. Prism coupling method was introduced to investigate the refractive index distribution of waveguides. The propagation modes in waveguides were analyzed by end-facet coupling method. The software of Mode Solution was introduced to investigate the propagation properties of strip-loaded waveguide with the different structure parameters. The results of experiment and simulation would provide new ideas for the development of integrated optical devices. Then, the ZnO nano-cages and ZHDS nano-sheets were fabricated by laser ablation Zn metal in solution. The absorption and photoluminescence properties of nano-materials were analyzed. The results provide a new paradigm to obtain hollow like nano-cages and metal/organic compound structure directly from bulk materials and inspires deeper investigations for generation more complex structures by this strategy in the future. The main results of dissertation include the following:1. The theory simulations of stripe-loaded waveguide on single-crystal Lithium niobate thin filmFor z-cut lithium niobate with a given TiO2 thickness of 100 nm, the single mode condition of planar waveguide was researched at 1550 nm wavelength. The single mode condition of stripe-loaded waveguide was researched in the range of the single mode condition of LN planar waveguide, with the different parameters. The simulation results were consistent with the analysis by Marcatili’s method; the TM mode was leaky because of TM(?)TE mode coupling at the sides of strip. The results provide the basis for follow-up studies and the design of waveguide devices.Mode size and light intensity distribution of z-cut LNOI stripe-loaded waveguide were simulated by Mode Solution at 1550 nm wavelength. The results showed that the stripe-loaded waveguide has a strong confinement of light, and could be form small mode size. As the width of TiO2 stripe was about 1 μm, the minimum mode size was obtained for TE/TM mode. The mode size would be decreased, when the thickness of TiO2 stripe was increased. For TE mode and TM mode, when the TiO2 stripe thickness was less than 100 nm, the light power confined in the LN layer was more than 90%.2. The TiO2 thin film and single-crystal LN thin filmPlanar waveguide was fabricated in z-cut LN by directed oxidation of Ti in a furnace with an oxygen flow at 500℃. The fabricated surface guiding structure was characterized by prism coupling method. The refractive index and extinction coefficient of TiO2 film were measured by ellipsometry at 1550 nm wavelength. The result of refractive index was consistent with that by prism coupling method. Planar waveguide was fabricated in z-cut LNOI under the same condition, the concentration of Ti was analyzed by SIMS with a function of depth, the result showed that Ti did not diffuse significantly in z-cut LN thin film at 500℃.The surface guiding structure of LNOI was characterized by prism coupling method at 632.8 nm wavelength. The results of measured effective refractive index of LNOI were consistent with that simulated by Mode Solution. This indicates the characteristics of LN were perfectly retained by single-crystal LN thin film fabricated by bonding method.3. stripe-loaded waveguide on single-crystal LN thin filmStripe-loaded waveguide was fabricated in z-cut LNOI by directed oxidation of Ti in a furnace with an oxygen flow at 500℃, the thickness of LN and TiO2 was 660 nm and 95 nm, respectively. The propagation modes of stripe-loaded waveguides were analyzed by end-facet coupling method at 1550 nm wavelength, the measured results was larger than the simulation results. The propagation loss of the waveguide was measured by Fabry-Perot method; the possible causes of loss were analyzed.In order to decrease the propagation loss of waveguides, the single mode condition of waveguides was investigated. For TM mode, when the thickness of LN thin film is less than 570 nm, there is only one mode in the waveguides. In order to decrease the propagation loss caused by mode coupling, we decreased the thickness of LN thin film and selected LNOI with 500 nm thick LN thin film. Low-loss stripe-loaded waveguide was fabricated in z-cut LNOI by directed oxidation of Ti in a furnace with an oxygen flow at 500℃, the thickness of LN and TiO2 was 500 nm and 100 nm, respectively. The propagation modes of stripe-loaded waveguides were analyzed by end-facet coupling method at 632.8 nm wavelength. The propagation loss of the waveguide was measured by Fabry-Perot method at 1550 nm wavelength, for TE mode, the loss was 1.72 dB/cm; for TM mode, the loss was 0.15 dB/cm, suggesting potential applications in near infrared communication.4. Y-branch waveguide on single-crystal LN thin filmThe relation between the bend loss and the bend radius was analyzed by Mode Solution, when the radius of the bend waveguide was greater than 500 μm, the bend loss could be negligible. The transmission of Y-junction was analyzed by Rsoft, when the thickness of TiO2 stripe was about 70 nm, the transmission of Y-junction reached to the largest value, stripe-loaded Y-junction waveguide was fabricated in x-cut LNOI by directed oxidation of Ti in a furnace with an oxygen flow at 500℃ the transmission of the Y-junction was analyzed by end-facet coupling method at 1550 nm wavelength. When the width of waveguides was 2 μm, the Y-junction exhibited larger transmission (80%-94%) under the condition of TE and TM polarization.5. Synthesis of novel nano-materialsZnO porous nano-cages with hollow spaces were successfully compounded by simply using laser ablation of Zn target in liquid medium containing deionized water and ammonia (Vwater:Vammonia=7:1-5:1). The ammonia concentration played a critical role for the final nano-particles. The ZnO nano-cages are of fine structures with low adsorptive capacity in UV-Vis region. Taking advantage of enough/excess amount of sodium dodecylsfulfate (SDS), the tightly packed and highly ordered layers of DS functional groups were generated and which resulted the crystallization of metallo-organic ZHDS nano-sheets over the plasma created by laser ablation of Zn target. Compared with ZnO nanoparticles, the novel free-standing two-dimensional ZHDS nano-sheets exhibited excellent fluorescence emission at 417 nm. It has been proved that the hydroxyl (-OH) groups should play the main role for the outstanding photoluminescence in the blue-band region.